This invention relates generally to gas turbines and in particular to methods and means for operating the turbine to reduce undesirable emissions, such as nitrogen oxides (NO.sub.x), unburnt hydrocarbons, and carbon monoxide (CO) from a gas turbine during changing load conditions.
Gas turbines coupled to electric generators are commonly used in power generation service. As gas turbines are capable of being quickly started and brought up to speed for operational loading, such units are generally considered more effective to use to handle grid peak loads (that is power demand spikes above a more constant grid base load) than steam turbine systems. Thus gas turbines are commonly used in a changing-load environment, in which they must respond to numerous increases and decreases in electric power demand.
It is desirable to optimize operation of a gas turbine in order to reduce undesirable emissions from the combustion process in the turbine. For example, the gas turbine combustion process results in generation of, among other things, nitrogen oxides (NO.sub.x), unburnt hydrocarbons, and carbon monoxide (CO). Such undesirable emissions can be minimized through control of the turbine's reaction zone temperature; this temperature in turn is well correlated with the fuel-air ratio (FAR) of the combustible mixture being fed into the combustion chamber of the turbine. Maintaining an optimal FAR thus minimizes undesirable emissions from the turbine. Further, maintaining an optimal FAR is also beneficial in optimizing the operability of the combustion system of the gas turbine.
Current gas turbine control systems typically employ a decentralized control strategy in which fuel supply to the turbine and air supply to the turbine are controlled by reference to different measured turbine performance parameters. Such a decentralized system results in different response times in the fuel control loop and in the air control loop, with a resultant variation of the FAR during transient load conditions, and a resultant increase in undesirable emissions and internal combustion acoustics.
For example, in a typical gas turbine controller fuel supply to the turbine is controlled primarily via a feedback loop that seeks to match turbine power output with the electrical load demand on the generator driven by the turbine. This feedback is typically through turbine speed, with a speed error signal (that is variation of the measured turbine speed with a reference (or set point) value) being processed to increase or decrease fuel supply to the turbine as appropriate. Air supply to the turbine in such a system is typically determined by the compressor airflow geometries which are controlled based on the error between actual turbine exhaust temperature and a reference value for the exhaust temperature; the compressor inlet guide vanes are positioned to increase or decrease air flow into the turbine as necessary to obtain the optimal exhaust temperature. Thus, a change in load on the turbine results in a faster response in adjustments to the fuel flow to the turbine than in adjustment to the air flow, which is not changed until the effect of the fuel flow change manifests itself as a change in the exhaust temperature of the turbine.
Even in steady state operation (no load demand changes), noise in the control system (such as variations in sensed turbine speed or variations in the exhaust temperature) or grid frequency changes can result in the control system frequently changing fuel flow and air flow.
In gas turbine control systems commonly in use, efforts to reduce FAR variations resulting from the lag between fuel supply control and air supply control (that is, fuel leads air in the current control system) have generally been unsuccessful. For example, attempts to reduce the lag by increasing the gain on the air flow loop, reducing the gain on the fuel loop, or both, generally puts the control system into a response region which is highly oscillatory and closer to the instability region.
It is thus an object of this invention to provide a method and apparatus for control of a gas turbine to minimize undesirable emissions of a gas turbine during steady state or transient operation.
It is a further object of this invention to provide a method and apparatus for control of a gas turbine that maintains a substantially constant fuel-air ratio in the combustible mixture being fed to the turbine, especially during transients in turbine load.
It is a further object of this invention to coordinate gas turbine fuel supply (that is, fuel flow) and air supply(that is, air flow) control signals so as to more closely couple the signals to maintain the turbine fuel-air ratio within a desired range to reduce undesirable emissions and to improve turbine performance and operability through increasing the operational range and service life of the turbine combustor.